1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of configuring secondary cells and related communication device.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has developed a universal mobile telecommunications system (UMTS) which adopts a wideband code division multiple access (WCDMA) as a wireless air interface. In the UMTS, a radio access network known as a universal terrestrial radio access network (UTRAN) includes multiple Node-Bs (NBs) for communicating with multiple user equipments (UEs). The WCDMA provides high frequency spectrum utilization, universal coverage, and high-speed multimedia data transmission which are beneficial for the UEs and the NBs of the UMTS. Furthermore, a long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard is now being developed by the 3GPP as a successor of the UMTS, to further enhance performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple UEs, and communicates with a core network including a mobility management entity (MME), serving gateway, etc for NAS (Non Access Stratum) control.
The main purpose of a handover is to maintain an ongoing connection which may be used for handling voice or data while a UE continuously moves. Since the UE may occasionally moves from the coverage of a source eNB to the coverage of neighboring eNBs, the ongoing connection must be transferred correspondingly to maintain the ongoing connection. Based on the measurement reports provided by the UE which include information of signal quality related to the source eNB and the neighboring eNBs, the source eNB determines whether the handover should be initiated. The source eNB performs the handover by sending the UE radio resource control (RRC) context information to the neighboring eNBs and receiving corresponding radio resource configurations from the neighboring eNBs. According to the radio resource configurations, the source eNB transmits a handover command to the UE, and the UE performs the handover including a radio access channel (RACH) procedure to a target eNB according to the handover command. Please note that, there is no dedicated handover command in the LTE system, a RRCConnectionReconfiguration message including a mobilityControlinfo is treated as the handover command to trigger the UE to perform the handover.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (COMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
For bandwidth extension, a carrier aggregation (CA) is introduced to the LTE-A system by which two or more component carriers are aggregated to achieve a wider-band transmission. Accordingly, the LTE-A system can support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 component carriers, where bandwidth of each component carrier is 20 MHz and is backward compatible with 3GPP Rel-8. The LTE-A system supports the CA for both continuous and non-continuous component carriers with each component carrier limited to a maximum of 110 resource blocks. The CA increases bandwidth flexibility by aggregating the non-continuous component carriers.
When the UE is configured with the CA, the UE is allowed to receive and transmit data on one or multiple component carriers to increase the data rate. In the LTE-A system, it is possible for the eNB to configure the UE different numbers of UL and DL component carriers which depend on UL and DL aggregation capabilities, respectively. Moreover, the component carriers configured to the UE necessarily consists of one DL primary component carrier (PCC) and one UL primary component carrier. Component carriers other than the primary component carriers are named UL or DL secondary component carriers (SCCs). The numbers of UL and DL secondary component carriers are arbitrary, and are related to the UE capability and available radio resources. Further, a cell operating on the primary component carrier is termed a primary cell, and a cell operating on the secondary component carrier is termed a secondary cell. Therefore, when the UE is configured with multiple component carriers, it implies that the UE is configured with multiple cells corresponding to the multiple component carriers.
On the other hand, a handover may occur between different radio access technologies, i.e., an inter-RAT (radio access technology) handover. For example, the UE may be handed over from an eNB of the LTE system to an eNB of the LTE-A system or the UMTS, and vice versa. In this situation, functional differences between the different radio access technologies should be taken into consideration when the inter-RAT handover happens. Further, when the UE is handed over from the eNB of the LTE-A system to the eNB of the LTE system or the UMTS (or an even earlier system), it is not known how to deal with the secondary cells, since the eNB of the LTE system or the UMTS does not support multiple component carriers and thus multiple cells. Besides, when the UE is handed over from the eNB of the LTE system or the UMTS to the eNB of the LTE-A system, the eNB of the LTE-A system cannot allocate the multiple component carriers to the UE immediately, since the eNB of the LTE system or the UMTS does not support the CA and can not provide related measurement results to the eNB of the LTE-A system. In other words, the eNBs and the UE may not operate normally and efficiently, when the inter-RAT handover happens. Therefore, signalings and protocols related to the inter-RAT handover must be designed such that the UE and the eNB can operate normally, and achieve their best performance by using the resources (e.g. component carriers and/or power) efficiently.